The energy output of a wind turbine depends on the rotor diameter. However, the mechanical stresses of the elements of the turbine structure (such as rotor, nacelle, and tower) increase quadratically with the rotor diameter. This is a serious drawback, especially when the wind speed is high, because larger rotors normally require lowering the switch-off speed, i.e., the wind speed over which the turbine stresses become unacceptable and the rotational speed of the rotor, and hence the turbine output, has to be decreased. Thus, it is important to control the rotor speed in order to limit the turbine structure fatigue.
The conventional current wind turbine design mindset treats the wind turbine structure and the wind turbine controller as separate entities. The controller normally is used to maximize power production below rated wind speed, and in higher wind speeds it maintains constant rotor velocity and power output. The turbine mechanical structure is then designed to support a fixed lifespan subject to the predicted lifetime loads. Unfortunately, during the operational life of a wind turbine, it may be discovered that the mechanical fatigue loads are either higher than predicted or the material fatigue properties are not as durable as anticipated, resulting in a shorter than expected lifetime for the turbine.
US Patent Application Publication No. US 2003/0127862 A1, published Jul. 10, 2003 in the name of Weitkamp, which is hereby incorporated by reference to the same extent as though fully disclosed herein, discloses a control system for a wind power plant that comprises sensors for the detection of measurement values, such as rotor speed, pitch angle, or wind speed, which are used for direct or indirect quantification of the current loading and stress of the turbine occurring in dependence on the local and meteorological conditions. An electronic signal processing system is also provided, operative to the effect that the power reduction required in the optimized condition of the wind power plant will be restricted to obtain optimum long-term economical efficiency under the current operating conditions, both in cases of winds in the range of the nominal wind velocity and in cases of high wind velocities.
In the art, the detected measurement values, or states, are processed into statistical data. See, for example, “The Statistical Variation of Wind Turbine Fatigue Loads” by Kenneth Thomsen, Risø National Laboratory, Roskilde, Denmark, September 1998, which is incorporated herein by reference to the same extent as though fully disclosed herein. The statistical operating data are converted into statistical stress data from which a stress distribution is derived. The control process is performed when the local or meteorological conditions undergo a relatively large change, or after a turbine cycle (hysteresis loop) is completed. At these times, which typically are on the order minutes or hours or even days, optimized values of the operational parameters or states of the turbine are set according to the optimized statistical data. In this way, statistically averaged aspects of the turbine structure are considered in the control of the turbine. However, there remains a risk that mounting damage may go undetected until it has severely curtailed the lifespan of the wind turbine, or, on the other hand, the wind turbine may be running for significant periods at below the optimum power level, either of which adversely affects the economics of the turbine. Thus, a wind turbine control system that provides for optimizing the economics of the turbine both with respect to power output and turbine structure would be highly desirable.